1. Field of the Invention
Implementations of various technologies described herein generally relate to seismic data acquisition, particularly the assessment of the impact of sensor locations on output trace quality.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
In a typical seismic survey, a plurality of seismic sources, such as explosives, vibrators, airguns or the like, may be sequentially activated at or near the surface of the earth to generate energy which may propagate into and through the earth. The seismic waves may be reflected back by geological formations within the earth. The resultant seismic wavefield may be sampled by a plurality of seismic sensors, such as geophones, hydrophones and the like. Each sensor may be configured to acquire seismic data at the sensor's location, normally in the form of a record or trace representing the value of some characteristic of the seismic wavefield against time. The acquired seismic data may be transmitted wirelessly or over electrical or optical cables to a recorder system. The recorder system may then store, analyze, and/or transmit the data. This data may be used to detect the possible presence of hydrocarbons, changes in the subsurface and the like.
Seismic data contains noise signals as well as the desired seismic reflection signals. Noise may interfere with the interpretation of the seismic signals and degrade the quality of the subsurface images obtained by processing the recorded seismic data. It may therefore be desirable to suppress or attenuate the noise that may be present in the recorded seismic data before processing the data. One method of noise attenuation that may be used in seismic acquisition is the process of analog group-forming. In this process, sensors may be hard-wired into groups or arrays of sensors. Each sensor array may produce an output trace that is the normalized sum (arithmetic average) of all traces acquired by the sensors in that array. However, high frequency components of some seismic events (for example, seismic events arising from dipped, non-horizontal reflectors) may be erroneously attenuated, while the low frequency components of ground-roll noise may not be attenuated.
Recently, single sensor acquisition (also known as point receiver recording) of seismic data has become possible. In single sensor acquisition, the sensors are not hard-wired into groups; rather, individual data traces recorded by each sensor may be recorded for processing. It may still be desirable to sum traces acquired by more than one sensor, but now the summation may be performed during the processing stage by summing any combination of the individual, digitized traces. Hence, this process may commonly be referred to as digital group-forming (DGF). Single sensor acquisition may allow the use of DGF for improved noise attenuation and signal preservation.
Typically, DGF includes the application of a multi-dimensional filter that may require regular sampling of the seismic data. Regular sampling in this context may include spatial sampling of seismic data at locations arranged on a regular grid such as a grid that is rectangular, square, hexagonal or the like. However in an actual seismic survey, the sensor locations may be spatially irregular and not on a regular grid due to obstacles, terrain and the like. Therefore, filters designed with regular theoretical data may produce output traces with distortion and noise leakage when applied to actual, irregularly sampled seismic data. It may be desirable to have a method to measure the quality of the output traces to determine if the actual, irregular sensor arrangement used in the seismic survey is producing acceptable results or introducing noise, aliasing and other errors.